Reduction of soluble contaminants in lean solvent

ABSTRACT

In a solvent extraction process wherein a mixed hydrocarbon feed is contacted in a primary extraction zone under extraction conditions with a specified water soluble primary solvent selective for aromatic hydrocarbons thereby producing a nonaromatic raffinate stream and an aromatic-rich solvent stream, the aromatic-rich solvent stream is separated in a separation zone to provide an aromatic product and a lean primary solvent stream containing hydrocarbon contaminant having a boiling point greater than the end boiling point of the aromatic product, and the lean primary solvent stream is passed to the extraction zone as the specified primary solvent, a method for reducing the concentration of soluble hydrocarbon contaminant in the lean solvent which comprises contacting a portion of lean primary solvent and at least a portion of the nonaromatic raffinate in a secondary extraction zone under extraction conditions including the presence of a secondary aqueous solvent. The secondary extraction zone produces a nonaromatic raffinate containing the hydrocarbon contaminant having substantial freedom from the primary solvent. The secondary extraction zone further produces a stream of rich secondary aqueous solvent containing the primary solvent having reduced concentration of the hydrocarbon contaminant, and the rich secondary aqueous solvent is separated to produce a lean primary solvent having reduced hydrocarbon contaminant concentration. The method has particular application to aromatic extraction processing wherein the primary solvent is a sulfolane-type chemical solvent, or a polyalkylene glycol solvent, or any other typically utilized water soluble solvent selective for aromatic hydrocarbons.

United States Patent Van Tassell- 1451 Feb. 15, 1972 541 REDUCTIONOF SOLUBLE CONTAMINANTS IN LEAN SOLVENT [72] Inventor: Harry M. Van Tassell, Arlington Heights,

Ill. a

[73] Assignee: Universal Oil Products Company, Des

Plaines,lll.

[22] Filed: June 18, 1970 21 Appl. No.: 47,501

[52] U.S. CL ..208/321, 260/674 SE 3,551,327 12/1970 Kelly et al...... ..208/321 Primary Exdminer-J-lerbert Levine Attomey-James R. l-loatson, Jr. and Glen P. Winton [57] ABSTRACT In a solvent extraction process wherein a mixed hydrocarbon feed is contacted in a primary extraction zone under extraction conditions with a specified water soluble primary solvent selective for aromatic hydrocarbons thereby producing a nonaromatic raffinate stream and an aromatic-rich solvent stream, the aromatic-rich solvent stream is separated in a separation zone to provide an aromatic product and a lean primary solvent stream containing hydrocarbon contaminant having a boiling point greater than the end boiling point of the aromatic product, and the lean primary solvent stream is passed to the extraction zone as the specified primary solvent, a method for reducing the concentration of soluble hydrocarbon contaminant in the lean solvent which comprises contact- 7 ing a portion of lean primary solvent and at least a portion of the nonaromatic raffinate in a secondary extraction zone under extraction conditions including the presence of a secondary aqueous solvent. The secondary extraction zone produces a nonaromatic rafl'mate containing the hydrocarbon contaminant having substantial freedom from the primary solvent. The secondary extraction zone further produces a stream of rich secondary aqueous solvent containing the primary solvent having reduced concentration of the hydrocarbon contaminant, and the rich secondary aqueous solvent is v separated to produce a lean primary solvent having reduced hydrocarbon contaminant concentration. The method has particular application to aromatic extraction processing wherein the primarysolvent is a sulfolane-type chemical sol- 1 vent, or a polyalkylene glycol solvent, or any other typically utilized water soluble solvent selective for aromatic hydrocarbons.

15 Claims, 1 Drawing Figure Ext/gator Column Extractive DisIi/loI/on Column Aromatic Recovery Steam BACKGROUND OF THE INVENTION The present invention relates to the solvent extraction of aromatic hydrocarbons from a hydrocarbon charge stream containing mixed hydrocarbon species. More particularly, the present invention relates to a method for the control of the concentration of soluble contaminants in the lean solvent utilized in an aromatic extraction process. More specifically, the present invention relates to an improved process for control of the concentration of soluble hydrocarbon contaminants in the lean solvent, where the contaminant hydrocarbons have a boiling point greater than the end boiling point of the high purity aromatic product which is distilled from the rich solvent of the extraction process.

In the aromatic extraction process which is typical of the present invention, the aromatic-containing hydrocarbon feed stock is passed to an extraction zone which may comprise a tower containing suitable packing such as Berl Saddles or Raschig Rings, or a tower containing suitable trays,'or a rotating disc contactor (RDC). The feed stock is contacted therein with a lean solvent composition under conditions sufi'lcient to produce a nonaromatic raffinate and an aromatic-rich solvent. The rich solvent composition leaving the extraction zone is passed to a rich solvent separation zone which typically comprises one or more fractionation columns which are operated in a manner sufficient to remove nonaromatics from the rich solvent, and to recover a high purity aromatic extract and the final lean solvent composition. The nonaromatic fraction which is removed is normally returned to the extraction zone to provide a nonaromatic reflux. Because the solvent chemicals typically utilized are chemically unstable at elevated temperatures, the aromatic extract is normally removed from the solvent composition with the assistance of steam stripping under operating conditions which minimize thermal decomposition of the solvent chemical. The aromatic extract is 'separated from the stripping steam condensate, and then passed into a fractionation train wherein the extract is separated into its aromatic constituents. The final lean solvent is withdrawn from the rich solvent separation zone and returned to the extraction zone.

A typical solvent which is utilized in commercial aromatics extraction and which may be processed in accordance with the practice of this invention is a solvent of the sulfolane type. The solvent possesses a five-membered ring containing one atom of sulfur and four atoms of carbon with two oxygen atoms bonded to the sulfur atom of the ring. Generically, the sulfolane type solvents may be indicated as having the following structural formula:

wherein R,, R R and R. are independently selected from the group comprising a hydrogen atom, an alkyl group having from one to ten carbon atoms, an alkoxy radical having from one to eight carbon atoms and an arylalkyl radical having from one to twelve carbon atoms. Other solvents which may be included within this process are the sulfolenes, such as 2-sulfolene or 3-sulfolene which have the following structures:

Other typical solvents which have a high selectivity for separating aromatics from nonaromatic hydrocarbons and which may be processed within the scope of the present invention are 2-methylsulfolane, 2, 4-dimethylsulfolane, methyl-2- sulfonyl ether, N-aryl-3-sulfonylamine, 2-sulfonyl acetate, diethylene glycol, various polyethylene glycols, dipropylene glycol, various polypropylene glycols, dimethyl-sulfoxide, N- methyl pyrollidone, etc.

The specifically preferred solvent chemical which is processed within the scope of the present invention is sulfolane, having the following structural formula:

DESCRIPTION OF PRIOR ART The typical solvent composition utilized in aromatic extraction processing comprises a mixture of water and one or more of the solvent chemicals herein noted. A particularly preferred solvent composition of the present invention comprises water and sulfolane. In extracting aromatic hydrocarbons from the hydrocarbon mixture, it is known that the paraffins are the least soluble followed in increasing order of solubility by naphthenes, olefins, dioletins, acetylenes, sulfur containing hydrocarbons, nitrogen containing hydrocarbons, oxygen containing hydrocarbons and aromatic hydrocarbons. It is the practice to regulate the solubility of the hydrocarbons within the solvent composition by varying the water content thereof. Thus, by adding more water to the solvent, the solubility of all components in the hydrocarbon mixture is decreased but the solubility difference between components (selectivity) is increased. The net effect is to decrease the number of contacting stages required to achieve a given purity of aromatic extract, or to increase the resulting purity of the aromatic extract when the number of contacting stages is held constant.

The presence of water in the solvent composition provides a further processing benefit in that it introduces a relatively volatile materialinto the fractionation system wherein the aromatic extract is separated from the rich solvent composition. The water of the solvent composition is vaporized at least in part to provide assistance in stripping all traces of nonaromatic hydrocarbons out of the aromatic-rich solvent, and to provide assistance in stripping the aromatic extract out of the final lean solvent.

It is, therefore, the practice to provide that the solvent composition contain from about 0.1 percent to about 20 percent wt. of water. When the solvent composition comprises chemical sulfolane, it is preferable that the solvent composition contain from about 0.1 percent to about 1.0 percent of water, while a solvent composition comprising a polyalkylene glycol preferably contains from about 6 percent to 15 percent of water.

As is known by those skilled in the art, the nonaromatic raffinate which leaves the extraction zone will contain some solvent. The solvent may be present in the raffinate partly as a soluble constituent in low concentration and partly as an entrained dispersion of free solvent phase due to the turbulence within the extraction zone. Because the typical solvent compositions which are utilized in aromatic extraction processing are water soluble, it is the practice to extract the solvent which is contained in the nonaromatic raffinate stream by contacting this raffinate stream with an aqueous stream in a subsequent extraction means. The extraction of the solvent from the raffinate utilizing water may be undertaken in any suitable liquid- The raffinate which then leaves the aqueous extraction zone or water wash" zone, will be substantially free of the solvent composition but the aqueous stream containing the recovered solvent will normally contain some nonaromatic hydrocarbons. The nonaromatic hydrocarbons are contained within the aqueous stream both as a soluble constituent of the aqueous stream and as a free hydrocarbon phase which is dispersed within the solvent-containing aqueous stream. This dispersion of microdroplets of nonaromatic hydrocarbons is entrained within the aqueous phase because of the turbulence which is normally experienced within the raffinate water wash zone.

This solvent-containing aqueous stream is normally sent back to the rich solvent separation zone to provide at least a part of the stripping steam which is utilized in separating the aromatic extract from the rich solvent. As the aqueous stream is generated into stripping steam, the solvent contained therein is thus recovered as a part of the final lean solvent which remains when the extract has been stripped out of the rich solvent. However, since this aqueous stream contains a substantial portion of nonaromatic hydrocarbons, it is undesirable to utilize this stream for direct generation of stripping steam since the nonaromatic raffinate contained therein would be vaporized in the stripping zone and thereby contaminate the resulting high purity aromatic extract. To avoid any contamination of the aromatic extract by nonaromatic constituents contained within the aqueous stream, it is therefore, typically the art to pass this aqueous stream to a prior distillation column or water still." The water still produces an overhead fraction comprising the nonaromatic raffinate constituents and water, and a bottoms fraction comprising water and solvent having substantial freedom from nonaromatic contaminants. This bottoms fraction is then passed to the extract recovery column in order to provide at least a part of the stripping steam utilized therein, while the nonaromatic overhead fraction may be passed to the extraction zone as a part of the nonaromatic extractor reflux.

It is well known by those skilled in the art, that the solvent chemicals utilized in the typical aromatic extraction process are thermally unstable. The instability is not pronounced, however, and only becomes evident upon prolonged recycling of the solvent, whereupon the accumulation of decomposition products becomes evident. Additionally, it is known that the rate of decomposition of the aromatic-selective solvent increases with increasing temperature. Accordingly, it is the practice with sulfolane solvent systems to set a maximum processing temperature of 350 F., while in diethylene glycol solvent systems and in triethylene glycol solvent systems it is the practice to set a maximum processing temperature of 380 F. Heat exchanger skin temperatures normally are held at a maximum of 450 F. to 500 F. for these specific solvent systems. Thus, it is the practice in the art to define such processing temperatures as being the point of thermal instability, although it is known that there is some trace decomposition occuring below such temperature levels.

It is known that the solvent decomposition results in the production of acidic organic deterioration products as well as polymerization products having a resinous character. The presence of organic acid within the aqueous solvent is known to cause corrosion of the steel equipment utilized in the process, and it is therefore the practice to add organic amine compounds to the solvent composition as corrosion inhibitors. Because of the basic characteristic of the amine inhibitors, these materials react with the acidic solvent decomposition products to form amine salts and amides at the temperature conditions utilized in the aromatic extraction process. With continued circulation of the solvent composition within the aromatic extraction process, there eventually occurs an accumulation of such relatively nonvolatile contaminants. This accumulation of nonvolatile contaminants results in the eventual precipitation of tarry insoluble deposits on the interior surfaces of the processing equipment, resulting in reduced heat transfer efficiency due to fouling of heat exchangers, and resulting in reduced separation efficiencies due to fouling of extractor decks and fractionating column trays.

It is, therefore, the practice in aromatic extraction processes to withdraw from the lean solvent recycle stream, a small slip stream of the lean solvent for solvent regeneration and recovery of a clean lean solvent not containing the relatively nonvolatile contaminants. The withdrawal rate is normally sufficient to provide that the entire solvent inventory of the aromatic extraction process is passed through the solvent regeneration system once every five to ten days. In this manner, the relatively nonvolatile contaminants never accumulate to a sufficiently high concentration to cause deposition of tarry insoluble sludge which is otherwise encountered within the solvent circulating system. The solvent regeneration system normally comprises a distillation column which is operated under maximum vacuum in order to minimize the vaporization temperature of the thermally unstable solvent chemical. The distillation column produces a solvent vapor containing unreacted organic amine inhibitor and water. This vapor is removed overhead, condensed and returned to the aromatic extraction process as a clean lean solvent liquid.

in the aromatic extraction process which is typical of the present invention, the aromatic containing feed stock will comprise a liquid hydrocarbon mixture having a sufficiently high concentration of aromatic hydrocarbons to economically justify the recovery of aromatics. Normally, the aromatics are present in the feed stock in a concentration of at least 25 percent by weight. The suitable carbon number range of the typical feed stock is from about 6 carbon atoms per molecule to about 20 carbon atoms per molecule. One source of a preferred feed stock is the depentanized reactor effluent from a naphtha reforming process unit. Another preferred source is the liquid byproduct gasoline from a pyrolysis processing unit, wherein the byproduct gasoline has been hydrotreated to saturate diolefins and olefins, and to remove sulfur and nitrogen contaminants.

More typically, the hydrocarbon feed stock which is processed in the aromatic extraction process will comprise a gasoline fraction containing aromatic and nonaromatic hydrocarbon species having from 6 to about 10 or 12 carbon atoms per molecule. When such a hydrocarbon feed stock is processed in the extraction zone, it is usually desired to maximize the recovery of benzene, toluene and xylenes from the original hydrocarbon mixture. Since the lean solvent has a high selectivity for aromatic hydrocarbons, any attempt to maximize such aromatic recovery, will also cause the lean solvent to dissolve heavier aromatic hydrocarbons having nine or more carbon atoms per molecule. When the resulting rich solvent is passed to the subsequent fractionation zone, the fractionation must be conducted under conditions sufficient to produce a high purity aromatic fraction containing benzene, toluene, mixed xylenes and ethylbenzene, and having substantial freedom from aromatic hydrocarbons having nine or more carbon atoms per molecule. In addition, the distillation must be conducted under conditions sufficient to provide that the chemical solvent will not undergo a substantial amount of thermal decomposition due to elevated temperatures within the fractionation zone.

Accordingly, when the lean solvent composition is withdrawn from the fractionation zone for return to the extraction zone, the lean zone will contain a substantial amount of aromatic hydrocarbons having a boiling point greater than the end boiling point of the high purity aromatic fraction which is produced by the distillation of the rich solvent. This presence of heavy aromatic hydrocarbon in the lean solvent phase may have a deleterious effect on the operation of the extraction zone, since the presence of this dissolved heavy aromatic contaminant in the lean solvent reduces the solubility and selectivity characteristics of the solvent composition.

It is to be noted that although a portion of the lean solvent is typically passed by means of a slip stream to the solvent regeneration column for the distillation of a clean lean solvent from the heavy resinuous polymer which otherwise accumulates therein, such a solvent regeneration step does not remove the heavy aromatic contaminant which is contained in the lean solvent. The vapor pressure characteristics of the heavy aromatic contaminants are such that these hydrocarbons will pass overhead in the vapor which comprises the clean lean solvent, the unreacted amine inhibitor, and water. Only the heavy tarry resinous polymeric contaminants of the lean solvent slip stream are removed from the lean solvent in the solvent regenerating column, since they are not sufficiently volatile to pass out with the overhead vapor.

' SUMMARY OF THE INVENTION It is thereforean object of the present invention to provide an improved extraction process wherein high purity aromatics may be separated from an aromatic containing feed stock in an economical and facile manner.

it is a particular object of this invention to provide a method for the reduction of the concentration of soluble hydrocarbon contaminants contained in the lean solvent utilized in an aromatic extraction process.

it is a more particular object of the present invention to provide an improved extraction process for the recovery of high purity aromatic products, wherein the presence within the lean solvent, of soluble hydrocarbon contaminants having a boiling point greater than the end boiling point of the high purity aromatic product, is maintained at a reduced concentration.

These and other objects of the present invention, as well as the advantages thereof, will become apparent from the description which follows hereinbelow, and which is'made with reference to the accompanying drawing. The attached drawing is a simplified schematic flow diagram of one conventional aromatic extraction process wherein the present inven tion may be practiced.

In its broadest aspects, it has been determined that the ob-, jects of this invention may be achieved by bringing the lean solvent composition, or at least a portion thereof, into contact with a diluent material in which the lean solvent chemical is readily soluble, but in which the heavy hydrocarbon contaminant fraction is not readily soluble. 7

By the practice of the present invention, at least a portion of the lean solvent stream is contacted in a contacting zone with an aqueous phase under conditions sufficient to provide that the lean solvent chemical will pass into solution in the aqueous phase while a substantial portion of the heavy hydrocarbon contaminant contained therein will pass into a distinct free hydrocarbon phase. More particularly, in the practice of the present invention a portion of the lean solvent composition is contacted with at least a portion of the nonaromatic raffinate produced in the solvent extraction process, in a contacting zone in the presence of an aqueous phase under conditions sufficient to provide that the contacting zone will produce a nonaromatic raffinate stream having substantial freedom from the lean solvent chemical while containing heavy hydrocarbon contaminant, and additionally produce an aqueous stream containing the lean solvent having a reduced concentration of the heavy hydrocarbon contaminant.

Therefore in accordance with the practice of the present invention, a broad embodiment comprises an improved solvent extraction process, wherein a mixed hydrocarbon feed is contacted in a primary extraction zone under extraction conditions with a specified water-soluble primary solvent selective for aromatic hydrocarbons, thereby producing a nonaromatic raffinate stream and an aromatic rich solvent stream, the aromatic-rich solvent stream is separated in a first separation zone to provide an aromatic product and a lean primary sol-' vent stream containing hydrocarbon contaminant having a boiling point greater than the end boiling point of the aromatic product, and the lean primary solvent stream is passed to the primary extraction zone as the specified primary solvent, which improvement comprises: (a) passing a portion of the lean primary solvent stream into a secondary extraction zone; (b) passing at least a portion of the nonaromatic raffinate stream into the secondary extraction zone; (c) contacting the portion of lean primary solvent and the nonaromatic raffinate in the secondary extraction zone under extraction conditions including the presence of a secondary aqueous solvent; (d) withdrawing from the secondary extraction zone the nonaromatic raffinate containing hydrocarbon contaminant and having substantial freedom from the primary solvent; (e) passing a stream of rich secondary solvent containing the primary solvent having reduced concentration of the hydrocarbon contaminant, from the secondary extraction zone into a second separation zone; (f) separating the rich secondary solvent stream-in the second separation zone under conditions sufficient to provide a stream of secondary aqueous solvent and a stream of primary solvent having reduced concentration of the hydrocarbon contaminant; and, (g) passing at least a portion of the separated primary solvent into the primary extraction zone.

This broad embodiment, and other more particular embodiments, may now be more clearly understood by discussing the present invention in light of the accompanying drawing.

DESCRIPTION OF THE DRAWING In one specific embodiment illustrating the application of the invention process, a depentanized full boiling range catalytic reformate gasoline is passed into a liquid-liquid extraction vessel 11 via line 10. The hydrocarbon feed comprises aromatic and nonaromatic hydrocarbons having from 6 to about 12 carbon atoms per molecule. A lean solvent composition comprising water and a solvent chemical, such as a polyethylene glycol or chemical sulfolane, enters the top of extractor 11 via line 22. Additionally, a nonaromatic reflux stream enters the bottom of extractor 11 via line 17. The nonaromatic reflux stream typically comprises benzene, toluene, hexane, and heptane, and it is passed into the bottom of theextractor 11 at a rate sufficient to displace substantially all higher molecular weight nonaromatic hydrocarbons from a resulting rich solvent stream containing aromatic hydrocarbons and leaving the bottom of extractor 11 via line 18. A nonaromatic raffinate stream leaves the top of extractor 11 via line 13. This raffinate stream typically comprises a minor portion of high molecular weight aromatic hydrocarbons, and a minor portion of dissolved and entrained solvent chemical. The nonaromatic raffinate stream is passed to a cooler 14 wherein the temperature of the nonaromatic raffinate is reduced to F.-or less. The cooled nonaromatic rafi'mate' then passes via line 15 into a water wash tower 16. The processing of the nonaromatic raffinate within water wash tower 1 6 will be discussed in detail hereinafter.

The extraction zone, comprising the vessel 11, is normally operated at elevated temperature and at a sufficiently high pressure to maintain the feed hydrocarbon, the reflux hydrocarbon, and the lean solvent in the liquid state with little or no possibility of vaporization occurring within extractor 11. in order to enhance solubility, the temperature within extractor 11 will normally be from about 80 F. to about 400 F., and preferably from about F. to about 300 F. Suitable pressures are within the range of from about slightly superatmospheric pressure. up to about 400 p.s.i.g., and preferably from about 50 p.s.i.g. to about 150 p.s.i.g. It is preferable that the volume of hydrocarbon reflux introduced into the lower section of extractor 11 via line 17 be at least 10 percent by volume of the rich solvent phase leaving the bottom of the extractor in order to most effectively displace the higher molecular weight nonaromatic hydrocarbons from the solvent phase. Similarly, it is necessary to provide a sufficient input of lean solvent composition entering extractor 11 via line 12, to dissolve substantially all of the lower molecular weight aromatic hydrocarbons out of the nonaromatic raffinate leaving the extractor 11 via line 13.

The rich solvent stream leaving extractor 11 via line 18 is passed into an extractive distillation column 19. Extractive matic constituents in the solvent liquid. In order to accomplish this, a lean solvent stream enters the top of extractive distillation column 19 via line 20. Alternatively, the additional lean solvent may be passed into line 18 via line 20 in order to provide that a suitable mixture of rich solvent will pass into the extractive distillation column 19 at the same feed point so that the proper degree of vaporization occurs. The amount of additional solvent passed into the extractive distillation column 19 via line 20, either at the upper locus as shown in the drawing or in combination with the rich solvent of line 18, is maintained at a rate sufficient to retain a substantial portion of the lower boiling aromatic hydrocarbon species in the liquid phase while allowing all nonaromatic hydrocarbon and a portion of the lower boiling aromatic hydrocarbon species to pass out of the top of the column in the vapor phase via line 49. The extractive distillation column 19 is provided with a typical reboiler circuit comprising a liquid line 21, a heat exchanger 22, and a vapor return line 23.

The extractive distillation column 19 is operated at moderate pressures and at a sufficiently high reboiler temperature to drive all of the low boiling nonaromatic hydrocarbon out of the liquid phase. Typical extractive distillation column pressures are from atmospheric pressure up to about 100 p.s.i.g., although generally, the top of distillation column 19 is maintained at a pressure from about 1 p.s.i.g. up to about 20 p.s.i.g. The reboiler temperature of extractive distillation column 19 is dependent upon the composition of the feed stock entering the process process via line and the solvent composition. As noted hereinabove, at all times the temperature within the extractive distillation column 19 must be maintained below the level of thermal instability of the solvent chemical. Accordingly, then, the reboiler temperature of column 19 is maintained below 350 F. when chemical sulfolane is the solvent utilized, and below 380 F. when a polyethylene glycol solvent is utilized.

Referring again to the drawing, the nonaromatic vapor which is withdrawn from the extractive distillation column 19 via line 49, typically comprises benzene, toluene, hexane, heptane, a minute portion of water vapor and a minor portion of solvent chemical vapor. An additional water vapor stream enters line 49 via line 44 from a source to be disclosed and discussed more fully hereinafter. The combined vapor stream passes along line 49 and enters a condenser 50 wherein the vapor constituents are cooled and condensed to the liquid state. The cooled liquid is passed from condenser 50 via line 51 into a separator vessel 52, typically maintained at a temperature of 100 F. or less. The liquid separates into a hydrocarbon phase and into an aqueous phase in separator 52. The hydrocarbon phase is withdrawn from separator 52 via line 17 and passed into the bottom of extractor vessel 11 as the nonaromatic reflux which was disclosed hereinabove. The aqueous phase comprising water and solvent chemical is withdrawn from separator 52 via line 53, and processed in a manner to be discussed hereinafter.

Referring now to the bottom of extractive distillation column 19, a rich solvent, containing aromatic hydrocarbons and having substantial freedom from nonaromatic hydrocarbons, is withdrawn from column 19 via line 24 and passed into an aromatic recovery column 25. Aromatic recovery column 25 is maintained under conditions sufficient to separate the desired aromatic hydrocarbon constituents from the rich solvent composition. Accordingly, column 25 is provided with a typical reboiler circuit comprising a liquid line 26, a reboiler heat exchanger 27, and a vapor return line 28. Additionally, stripping steam is passed into the bottom of aromatic recovery column 25 via line 29. The rate of steam stripping which is utilized in column 25 is sufficient to produce a high purity aromatic vapor passing from the top of aromatic recovery column 25 via line 32. The hot vapor, comprising the aromatic hydrocarbon product and stripping steam, passes into a condenser 33 wherein the vapor is condensed and cooled before passing into a phase separator 35 via line 34. Phase separator 35 is maintained at a temperature of 100 F. or less, and the hydrocarbon constituents are separated therein from the aqueous phase. One portion of the hydrocarbon phase contained in separator 35 is passed back to the top of column 25 via line 36 as reflux. A second portion of the hydrocarbon phase contained in separator 35 is withdrawn from the separator via line 37 as a high purity aromatic product typically comprising benzene, toluene, mixed xylenes, and ethyl-benzene. The aqueous phase is withdrawn from separator 35 via line 38 and processed in a manner which will be discussed in greater detail hereinafter.

As noted hereinabove, the aromatic recovery column 25 is operated under conditions sufficient to produce a high purity aromatic product as an overhead vapor stream and a lean solvent as a bottoms stream. These conditions not only include the presence of stripping steam entering the column via line 29, but typically will include operation at low pressures and sufficiently high temperatures to enhance the separation which is desired within the column. The choice of operating conditions depends upon the type of aromatic constituents being processed in column 25, as well as upon the solvent composition. Typically, the top of the aromatic recovery column 25 is maintained at a vacuum of from about to about 400 millimeters of mercury absolute. Such a low pressure must be used since the reboiler temperature must be maintained below the level of thermal decomposition for the solvent chemical being utilized. Accordingly, the reboiler temperature is maintained below 350 F. when chemical sulfolane is the solvent being utilized, and below 380 F. when a polyethylene glycol solvent is utilized.

Referring now to the bottom of aromatic recovery column 25, a lean solvent stream is introduced into the bottom of the column via line 30. This lean solvent stream is obtained from a source to be discussed fully hereinafter, and it combines with the lean solvent composition which results from the distillation occurring in the column 25, to provide a net lean solvent stream passing from the column 25 via line 12. Alternatively, the lean solvent of line 30 may be passed directly into line 12 via line 31 instead of passing into the recovery column 25.

The lean solvent composition in line 12 comprises the solvent chemical, water, and aromatic hydrocarbon constituents having a boiling point greater than the end boiling point of the high purity aromatic product which was withdrawn from column 25 via line 32. Additionally, the lean solvent of line 12 will contain trace products of thermal decomposition, which are typically removed in a solvent regeneration system, not shown. As the lean solvent composition flows along line 12, a first portion is withdrawn therefrom via line 20 and passed into the extractive distillation column 19 for processing in the manner which has been discussed hereinabove. The remaining portion of the lean solvent composition continues to flow along line 12 until a second portion is withdrawn therefrom via line 40 and passed into the water wash tower 16. The balance of the lean solvent composition of line 12 is passed into the top of extractor 11, wherein the solvent contacts the hydrocarbon feedstock in the manner which was discussed hereinabove.

Referring now to the water wash tower 16, the nonaromatic raffinate enters the bottom of the tower 16 via line 15 as noted hereinabove. Additionally, the lean solvent stream enters the tower via line 40, preferably at a locus intermediate to the top and the bottom of the water wash tower. The nonaromatic raffinate entering the bottom of water wash tower 16 typically comprises nonaromatic hydrocarbons with a minor concentration of aromatic hydrocarbons, but it also contains a minor amount of the solvent chemical which is contained in the raff'mate due to solubility considerations and due to entrainment of solvent phase which occurs as the raffinate leaves extractor 11 via line 13. The lean solvent composition entering the water wash tower via line 40 comprises the solvent chemical, water, and a minor amount of heavy hydrocarbons having a boiling point greater than the end boiling point of the net aromatic product fraction produced at the aromatic recovery column 25.

The nonaromatic raftinate of line and the solvent composition of line 40 are intimately contacted within the water wash tower 16 in the presence of an aqueous secondary solvent entering the top of the water wash tower via line 38. This aqueous secondary solvent comprises at least a portion of the water phase withdrawn from the separator 35 at the aromatic recovery column 25. A portion of the water withdrawn from separator 35 may be discharged as excess water from line 38 via line 39. The balance of the water leaving separator 35 is passed into the water wash tower 16 via line 38 in an amount sufficient to provide that substantially all solvent contained within the contacting zone, comprising tower 16, will pass into the aqueous phase.

A net nonaromatic raftinate stream leaves water wash tower 16 via line 41. Since the aqueous phase has a low solubility for hydrocarbon, the net raffinate stream contains a portion of the heavy aromatic hydrocarbon contaminant introduced into the water wash tower 16 in the lean solvent composition of line 40. However, the nonaromatic raffinate stream leaving via line 41 has substantial freedom from the solvent chemical. That is to say, the conditions within the water wash tower 16 are maintained such that, the net raffinate leaving the tower normally contains less than 100 p.p.m. of solvent chemical, and typically less than p.p.m. of solvent chemical.

Typically, water wash tower 16 is maintained at a temperature of 100 F. or less, although slightly higher temperatures may be utilized. Normally, the pressure within water wash tower 16 will be only that which is required to maintain all constituents contained therein in the liquid state. Typically, the pressure within column 16 will be from about 20 p.s.i.g. to about I00 p.s.i.g. or even higher.

A solvent-rich aqueous stream is withdrawn from water wash tower 16 via line 42. This solvent-rich aqueous stream contains substantially all of the solventchemical which entered water wash column 16 as the lean solvent slip stream of line 40, and as the dissolved and entrained solvent content of the nonaromatic raffinate stream entering via line 15. However, due to the conditions of aqueous extraction maintained within water washtower 16, the solvent dissolved within the aqueous phase contains a substantially reduced concentration of heavy aromatic contaminants having a boiling point greater than the end boiling point of the net aromatic product of line 37. This solvent-rich aqueous phase of' line 42 is combined with the aqueous phase of line 53 which also comprises the solvent chemical contained in a water stream as noted hereinabove. The resulting mixture of solvent chemical and water passes via line 42 into a water still 43.

Water still 43 is operated under conditions sufficient to vaporize a substantial portion of the water from the lean solvent. Accordingly, water still 43 is provided with a typical reboiler system comprising a liquid line 46, a reboiler heat exchanger 47, and a vapor return line 48. A first portion of the steam generated within water still 43 is withdrawn from the top of the column via line 44, and it is passed via line 44 into line 49, wherein it is combined with the overhead vapor of the extractive distillation column 19 as noted hereinabove. This overhead vapor stream is withdrawn via line 44 in order to remove all trace quantities of nonaromatic hydrocarbon contaminants which are inherently picked up in the aqueous streams leaving water wash tower l6 and separator 52 via lines 42 and 53, respectively. Typically, the steam passing out of water still 43 via line 44 will contain trace quantities of hexane and possibly heptane.

A clean stream product is withdrawn from an intermediate section of water still 43 via line 45. This clean stream, which is substantially free of nonaromatic contaminant, is passed into line 29 and thereby into the bottom of aromatic recovery column in order to provide at least a portion of the stripping steam which is utilized for the separation of the high purity aromatic product from the solvent within column 25.

In summary, the water still 43 is operated under conditions sufficient to generate steam while removing trace quantities of nonaromatic hydrocarbon via line 44, and to generate a clean stream passing via line 45 and line 29 into column 25. These operating conditions will comprise elevated temperatures and preferably a low pressure. Since 'the contaminated steam discharged from the top of water still 43 via line 44 passes into the overhead vapor line 49, water still 43 will typically operate at pressure ranges which are similar to that for the extractive distillation column 19. Preferably, the top of the water still 43 will operate at a pressure of from about 1 p.s.i.g. up to about 20 p.s.i.g., although higher pressures may be used. The reboiler temperature which is maintained within water still 43 will be that required to provide the amount of steam generation which is necessary. However, it is to be noted that the reboiler temperature must not exceed the level of thermal decomposition for the solvent chemical being utilized. Accordingly, the reboiler temperature of water still 43 is maintained below 350 F. when chemical sulfolane is the solvent utilized, and below 380 F. when a polyethylene glycol solvent is utilized. 1

Referring now to the bottom of water still 43, a net lean solvent stream is withdrawn therefrom via line 30. This lean solvent stream comprises the solvent chemical having a substantially reduced concentration of heavy aromatic hydrocarbons having a boiling point greater than the end boiling point of the net aromatic product of line 37. Additionally, this net lean solvent stream of line 30 contains a minor quantity of water. This lean solvent is typically passed via line 30 into the bottom of aromatic recovery column 25. Alternatively, the lean solvent is not passed into the bottomof column 25, but is pumped directly into line 12 via line 31. However, since the aromatic recovery column 25 nonnally operates at a pressure level substantially below the pressure of water still 43, it is preferably to pass the lean solvent of line 30 directly into the bottom of column 25 in order that the transfer out of column 43 may be accomplished by means of pressure drop only. without the utilization of a pump.

The following example is now presented to illustrate the effectiveness of the process of the present invention.

EXAMPLE In a specific operation illustrating the application of the inventive process, a depentanized and hydrotreated mixture of pyrolysis naphtha and coke oven oil was solvent extracted to produce nitration grade benzene, nitration grade toluene, and a mixed C aromatic fraction. This aromatic-rich hydrocarbon feed stock contained hydrocarbon species having from 6 to about 12 carbon atoms per molecule. V

Referring now to the attached drawing, the hydrocarbon feed stock entered the process via line 10 at a rate of 3,093 B.P.S.D. (barrels per stream day) or a rate of 424.34 mols./hr. (As used herein, the tenn moles per hour refers to pound moles per hour.) The feed stock was passed into extractor 11 via line 10 wherein it was contacted with a solvent composition comprising chemical sulfolane and water, which entered the extractor via line 12. Additionally, a reflux hydrocarbon stream entered the extractor via line 17. A net rich solvent fraction was processed through the extractive distillation column 19 and the aromatic recovery column 25 under conditions sufficient to produce a net aromatic product stream which was withdrawn from the process of the present invention via line 37 at a rate of 330.22 molsJhr. or 2,244 B.P.S.D. This net aromatic stream of line 37 comprised a high purity mixture of benzene, toluene, mixed xylenes, and ethylbenzene. Operating conditions maintained within aromatic recovery column 25 produced a lean solvent composition comprising chemical sulfolane and water which contained an equilibrium concentration of aromatic hydrocarbon contaminants having nine or more carbon atoms per molecule.

A nonaromatic raflinate stream was withdrawn from the top of extractor 11 via line 13 at a rate of 94.03 mols./hr. This nonaromatic raffinate stream contained 2.05 molsJhr. of arcmatic hydrocarbons and 0.67, molsJhr. of chemical sulfolane. The chemical sulfolane contained within the nonaromatic raffinate stream of line 13 was present therein due to the solubility of sulfolane in the raffinate and due to entrainment of a free solvent phase as the nonaromatic raffinate left the top of extraction 11.

The nonaromatic raffinate stream was passed into cooler 14 via line 13 wherein the temperature of the hot raffinate leaving the extractor 11 was reduced to a level below 100 F. The cooled nonaromatic raffinate was then passed via line 15 into the bottom of water wash tower 16. Simultaneously, a slip stream of lean solvent composition was passed into the water wash tower via line 40 at a rate of 160.0 mols./hr. This lean solvent stream contained 5.19 mols./hr of water and 0.83 mo1s./hr. of C aromatics. The balance of the lean solvent stream comprised chemical sulfolane. A wash-water stream was passed into the top of the water wash tower 16 via line 38 at a rate of 342.43 mols./hr.

As illustrated in the attached drawing, the lean solvent slip stream was passed into the water wash tower 16 via line 40 at a central section in the tower. This provided for an optimum degree of contacting of the lean solvent composition with the water phase and with the hydrocarbon phase. The water wash tower was operated at a temperature of less than 100 F. and at a pressure of about 60 p.s.i.g. Additionally, the water wash tower was operated with the hydrocarbon phase as the continuous phase and with the solvent-containing aqueous phase as the dispersed phase. A net raffinate product was withdrawn from water wash tower 16 via line 41 at a rate of 845 B.P.S.D. or 94.12 mols./hr. This net raffinate product comprised 2.81 mols./hr. of aromatic hydrocarbons, and 13 ppm. of chemical sulfolane was dissolved therein.

A rich wash water was withdrawn from the bottoms of water wash tower 16 via line 42 at a rate of 502.34 mols./hr. This stream comprised 347.62 mols./hr. of water, 154.65 mols./hr. of chemical sulfolane, and 0.07 mols./hr. of C aromatic hydrocarbons. Additionally, due to the turbulence experienced within the water wash tower and due to the solubility characteristics of the solvent-containing rich wash water, the wash-water stream ofline 42 contained a trace of nonaromatic hydrocarbon contaminant. The rich wash water of line 42 was combined with a separator water stream passing into line 42 via line 53 at a rate of 101.33 mols./hr. This separator water comprised 99.71 mols./hr. of water and 1.62 mols./hr. of chemical sulfolane, and it contained a trace of nonaromatic hydrocarbon which was dissolved in the aqueous phase during separation of the hydrocarbon and aqueous phases occurring within the receiver 52.

A resulting combined mixture of solvent-rich water was thereafter passed into the water still 43 via line 42 at a rate of I 603.67 mols./hr. This feed stream comprised 447.33 mols./hr.

of water, 156.27 mols./hr. of chemical sulfolane and 0.07 mols./hr. of C aromatics. Additionally, this water feed contained a trace of nonaromatic hydrocarbon comprising hexane. The water still 43 was operated at a reboiler temperature of 25 1" F. and at a reboiler pressure of 7.0 p.s.i.g.

A net overhead vapor was withdrawn from the water still 43 via line 44 at a rate of 44.34 mols./hr. This overhead vapor was predominantly pure steam containing a trace of nonaromatic hydrocarbon vapor. Additionally, a side cut vapor was withdrawn from the water still 43 via line 45 at a rate of 397.20 mols./hr. This side cut vapor comprised steam having substantial freedom from nonaromatic hydrocarbons. A net water still bottoms fraction was withdrawn from the lower portion of the water still 43 via line 30 at a rate of 162.13 mols./hr. The water still bottoms stream comprised 156.27 mols./hr. of chemical sulfolane, 5.79 mols./hr. ofliquid water, and 0.07 m0ls./hr. of C aromatic hydrocarbon.

PREFERRED EMBODlMENTS The method of operation for the present invention, as well as the advantages thereof, will now be readily apparent to those skilled in the art from the foregoing disclosure.

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In particular, the effectiveness of the present invention may be noted by comparing the hydrocarbon content of the lean solvent composition passing into the water wash tower 16 via line 40 with the hydrocarbon contaminant content of the lean solvent composition passing out of the water still 43 via line 30. It will be seen that a substantial reduction in the amount of heavy aromatic hydrocarbon having a boiling point greater than the end boiling point of the net aromatic product of line 37 occurred due to the preferential passage of the heavy hydrocarbon contaminant out of the aqueous solvent phase and into the net raffinate product passing from the water wash tower 16 via line 41.

Additionally, it will be noted that the chemical sulfolane content of the nonaromatic raffinate passing from the extractor 11 into the water wash tower 16 via line 15 was substantially fully recovered by the preferential solubility of the sulfolane solvent in the aqueous phase.

It is to be noted that the operating conditions which were disclosed in the above example are specific to that example, and that the scope of the present invention is not limited thereto. Those skilled in the art are able to readily ascertain the specific operating conditions which are required for any particular operation. Of course, these conditions will depend upon the type of solvent chemical utilized. That is to say, the operating conditions will depend upon whether the solvent is chemical sulfolane or a polyalkylene glycol or a similar solvent. Additionally, the operating conditions will depend upon the amount of the solvent chemical contained in the nonaromatic raffinate stream entering the water wash tower 16 via line 15, as well as in the concentration of aromatic hydrocarbon contaminant contained in the lean solvent slip stream entering the water wash tower via line 40.

Furthermore, it must be pointed out that the amount to heavy hydrocarbon contaminant contained in the lean solvent of the aromatic extraction process will be established at an equilibrium level which is dependent upon the operating conditions which are maintained in the aromatic recovery column 25. This equilibrium level of the heavy aromatic hydrocarbons in the lean solvent will depend upon how hard the rich solvent phase is stripped with steam in column 25. Additionally, it will depend upon the end boiling point of the type of aromatic hydrocarbon which is desired as the net aromatic product of line 37. Finally, it is to be noted that the equilibrium concentration of hydrocarbons in the lean solvent will be dependent upon the reboiler temperature maintained in column 25, and that this temperature is in turn limited by the level of thermal instability of the specific solvent chemical being utilized.

As noted hereinabove, the lean solvent slip stream of line 40 was passed into a central section of the water wash tower 16, the nonaromatic raffinate stream was passed into a lower section, and a wash-water stream was passed into the tower at an upper section. However, the method of contacting is not so limited. For example, the lean solvent slip stream could be combined with the nonaromatic raffinate stream ahead of the water wash tower and the resulting mixture could be passed into the water wash tower at a lower locus. Similarly, in the illustrative example the water wash tower was operated with the hydrocarbon phase as the continuous phase in the tower. Alternatively, the water wash tower could be operated with the aqueous phase as the continuous phase in the tower. The preferred method of operation of the water wash tower will, of course, depend upon the component analysis of the lean solvent slip stream and the component analysis of the nonaromatic raffinate, as well as the relative quantity of these two streams and the relative quantity of wash water which is available.

These and other modifications to the method of operation for the inventive process will be readily apparent to those skilled in the art from the foregoing teachings, and it must be realized that such modifications do not in any manner detract from the broadness of scope of the present invention.

In conclusion, it may now be summarized that a preferred embodiment of the present'invention resides in a process for allthe recovery of aromatic hydrocarbons from a mixed hydrocarbon charge stock containing nonaromatic hydrocarbons which comprises: (a) contacting the charge stock with a specified primary solvent in an extraction zone maintained under conditions sufficient to provide a first raffinate stream comprising nonaromatic hydrocarbons, and a rich solvent stream containing aromatic hydrocarbons; (b) passing the rich solvent stream into a first distillation zone; (c) passing stripping steam into the first distillation zone under conditions sufficient to provide a high-purity aromatic stream, a first aqueous stream, and a first lean solvent containing heavy aromatic hydrocarbon having a boiling point greater than the end boiling point of the high-purity aromatic stream; (d) passing a first portion of the first lean solvent from the first distillation zone into the extraction zone as a first part of said primary solvent specified; (e).passing at least a portion of the first raffinate stream, at least a portion of the first aqueous stream, and a second portion of the first lean solvent, into a contacting zone maintained under extraction conditions; (f) withdrawing from the contacting zone, a second rafiinate stream containing heavy aromatic hydrocarbon, and a second aqueous stream containing primary solvent having a reduced concentrationof heavy aromatic hydrocarbon; (g) passing the second aqueous stream into a second distillation zone maintained under conditions sufficient to provide at least a portion of the stripping steam, and to provide a second lean solvent having a reduced concentration of heavy aromatic hydrocarbon; (h)

passing at least a portion of the second lean solvent into the extraction zone as a second part of the primary solvent specified; and (i) recovering the high purity aromatic stream.

The invention claimed:

1. In a solvent extraction process wherein a mixed hydrocarbon feed is contacted in a primary extraction zone under extraction conditions with a specified water soluble primary solvent selective for aromatic hydrocarbons thereby producing a nonaromatic raffinate stream and an aromatic-rich solvent stream, said aromatic-rich solvent stream is separated in a first separation zone to provide an aromatic product and a lean primary solvent stream containing hydrocarbon contaminant having a boiling point greater than the end boiling point of said aromatic product, and said lean primary solvent stream is passed to said extraction zone as said specified primary solvent, the improvement which comprises:

a. passing a portion of said lean primary solvent stream into a secondary extraction zone; 7

b. passing at least a portion of said nonaromatic raffinate stream into said secondary extraction zone;

c. contacting said portion of lean primary solvent and said portion of nonaromatic raffinate in said secondary extraction zone under extraction conditions including the presence of a secondary aqueous solvent;

d. withdrawing from said secondary extraction zone said nonaromatic raffinate containing said hydrocarbon contaminant;

e, passing a stream of rich secondary solvent containing said primary solvent having reduced concentration of said hydrocarbon contaminant, from said secondary extraction zone into a second separation zone;

' f. separating said rich secondary solvent stream in said second separation zone under conditions sufficient to provide a stream of secondary aqueous solvent and a stream of primary solvent having reduced concentration of said hydrocarbon contaminant; and,

g. passing at least a portion of said separated primary solvent into said primary extraction zone.

2. Process of claim 1 wherein said hydrocarbon contaminant comprises an aromatic hydrocarbon.

3. Process of claim 1 wherein said separated primary solvent is passed into said first separation zone.

4. Process of claim 1 wherein said primary extraction zone comprises an extractive distillation zone.

5. Process of claim 1 wherein said primary solvent comprises a sulfolane-type chemical of the general formula:

wherein R,, R R and R are independently selected from the group consisting of a hydrogen atom, an alkyl group having from one to ten carbon atoms, an arylalkyl radical having from one to twelve carbon atoms, and an alkoxy radical having from one to eight carbon atoms.

6. Process of claim 5 wherein said primary solvent comprises sulfolane.

7. Process of claim 1 wherein said primary solvent comprises a sulfolene selected from the group consisting of 2-sulfolene and 3-sulfolene.

8. Process of claim 1 wherein said primary solvent comprises at least one polyalkylene glycol.

9. Process of claim 8 wherein said primary solvent comprises at least one of the group consisting of diethylene glycol, dipropylene glycol, and triethylene glycol.

10. Process for recovery of aromatic hydrocarbons from a mixed hydrocarbon charge stock containing nonaromatic hydrocarbons which comprises:

a. contacting said charge stock with a specified primary solvent in an extraction zone maintained under conditions sufficient to provide a first raffinate stream comprising nonaromatic hydrocarbons, and a rich solvent stream containing aromatic hydrocarbons;

b. passing said rich solvent stream into a first distillation zone;

c. passing stripping steam into said first distillation zone under conditions sufficient to provide a high-purity aromatic stream, a first aqueous stream, and a first lean solvent containing heavy aromatic hydrocarbon having a boiling point greater than the end boiling point of said high-purity aromatic stream;

. passing a first portion of said first lean solvent from said first distillation zone into said extraction zone as a first part of said primary solvent specified;

e. passing at least a portion of said first raffinate stream, at

least a portion of said first aqueous stream, and a second portion of said first lean solvent, into a contacting zone maintained under extraction conditions;

f. withdrawing from said contacting zone, a second raffinate stream containing heavy aromatic hydrocarbon, and a second aqueous stream containing primary solvent having a reduced concentration of heavy aromatic hydrocarbon;

g. passing said second aqueous stream into a second distillation zone maintained under conditions sufi'icient to provide at least a portion of said stripping steam, and to provide a second lean solvent having a reduced concentration of heavy aromatic hydrocarbon;

h. passing at least a portion of said second lean solvent into said extraction zone as a second part of said primary solvent specified; and,

i. recovering said high purity aromatic stream.

11. Process of claim 10 wherein said first raffinate stream contains primary solvent and said second raffinate stream has substantial freedom from primary solvent.

12. Process of claim 10 wherein second lean solvent is passed into said first distillation zone.

13. Process of claim 10 wherein said extraction zone comprises an extractive distillation zone.

14. Process of claim 10 wherein said primary solvent comprises sulfolane.

15. Process of claim 10 wherein said primary solvent comprises a polyalkylene glycol selected from the group consisting of diethylene glycol, dipropylene glycol, and triethylene glycol. 

2. Process of claim 1 wherein said hydrocarbon contaminant comprises an aromatic hydrocarbon.
 3. Process of claim 1 wherein said separated primary solvent is passed into said first separation zone.
 4. Process of claim 1 wherein said primary extraction zone comprises an extractive distillation zone.
 5. Process of claim 1 wherein said primary solvent comprises a sulfolane-type chemical of the general formula:
 6. Process of claim 5 wherein said primary solvent comprises sulfolane.
 7. Process of claim 1 wherein said primary solvent comprises a sulfolene selected from the group consisting of 2-sulfolene and 3-sulfolene.
 8. Process of claim 1 wherein said primary solvent comprises at least one polyalkylene glycol.
 9. Process of claim 8 wherein said primary solvent comprises at least one of the group consisting of diethylene glycol, dipropylene glycol, and triethylene glycol.
 10. Process for recovery of aromatic hydrocarbons from a mixed hydrocarbon charge stock containing nonaromatic hydrocarbons which comprises: a. contacting said charge stock with a specified primary solvent in an extraction zone maintained under conditions sufficient to provide a first raffinate stream comprising nonaromatic hydrocarbons, and a rich solvent stream containing aromatic hydrocarbons; b. passing said rich solvent stream into a first distillation zone; c. passing stripping steam into said first distillation zone under conditions sufficient to provide a high-purity aromatic stream, a first aqueous stream, and a first lean solvent containing heavy aromatic hydrocarbon having a boiling point greater than the end boiling point of said high-purity aromatic stream; d. passing a first portion of said first lean solvent from said first distillation zone into said extraction zone as a first part of said primary solvent specified; e. passing at least a portion of said first raffinate stream, at least a portion of said first aqueous stream, and a second portion of said first lean solvent, into a contacting zone maintained under extraction conditions; f. withdrawing from said contacting zone, a second raffinate stream containing heavy aromatic hydrocarbon, and a second aqueous stream containing primary solvent having a reduced concentration of heavy aromatic hydrocarbon; g. passing said second aqueous stream into a second distillation zone maintained under conditions sufficient to provide at least a portion of said stripping steam, and to provide a second lean solvent having a reduced concentration of heavy aromatic hydrocarbon; h. passing at least a portion of said second lean solvent into said extraction zone as a second part of said primary solvent specified; and, i. recovering said high purity aromatic stream.
 11. Process of claim 10 wherein said first raffinate stream contains primary solvent and said second raffinate stream has substantial freedom from primary solvent.
 12. Process of claim 10 wherein second lean solvent is passed into said first distillation zone.
 13. Process of claim 10 wherein said extraction zone comprises an extractive distillation zone.
 14. Process of claim 10 wherein said primary solvent comprises sulfolane.
 15. Process of claim 10 wherein said primary solvent comprises a polyalkylene glycol selected from the group consisting of diethylene glycol, dipropylene glycol, and triethylene glycol. 